The Fine Things are Always Hand Made
Scott 800 PM vs electromagnetic speakers
I have an 800 chassis that I bought some years ago without a speaker or cabinet. I'd like to get it working again but have some confusions about the speakers. I understand that some were PM and some electromagnetic and the ones with PM's have a resistor to add some impedance substitution for the coil in the EM version. Was there an electrical difference in the 2 versions? I'm assuming that the PM version utilized a choke in the power supply. If there was a difference in the electronics, how can I tell which version I have. Thanks in advance!
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Replies to This Discussion
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Permalink Reply by Norman S Braithwaite
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There were two versions of the Model 800B, early and late. The late version had two chokes on the power amplifier and a revised tube layout. Many of the early version were updated to add the additional choke. The early version had two tubes adjacent to the 6L6 output tubes whereas the late version had one tube adjacent to the 6L6 tubes. The revision included moving the voltage regulator tube away from the output tubes. The field coil on the 800B speaker was also used as a choke in the B+ supply. The PM speaker version simply had a power resistor mounted on the speaker basket to replace the DC resistance of the field coil. Going from recollection, the speakers were interchangeable.
Norman
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Permalink Reply by Daniel Kingloff
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So what is the native output impedance of the output transformers?
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Permalink Reply by Alex R. Whitaker
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Check the Scott Info Archive on this page.....it might have that information. Based on my experience with other brands, in that post-war era, it's probably 3.2 ohms to the speaker voice coil. Input would be typical 6L6 class B impedance, which can be found in any tube manual.
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I'll do it! Thanks
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Permalink Reply by David C. Poland
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The Scott 800-B is a complicated post war radio made more so with the extensive wiring for the station preset buttons running the motorized dial. And also because relays operate the power switch and the AM/FM switching. But properly restored is an impressive sounding radio.
FYI - the 800-B speaker's Jones plug includes a jumper (pin 1 to pin 2, as I recall) through which AC travels in route to the power switch - so the radio will not power up without the proper speaker plugged in, in as much as the field coil of the earlier production speakers is part of the power supply circuit. The later pm speakers were backwards compatible with earlier chassis, by virtue of the power resistor hidden inside the plastic bell cover. I mostly have seen the teal painted Jensens, but some had a Stephens and one other manufacturer I don't recall, all mentioned in the documentation.
The teal colored Jensen co-axial has turned up on eBay from time to time, so if you think you see one, someone here can probably confirm if it is the one for the 800-B. The model 800-B speakers are curve cone 15 inch with center mounted tweeter. The tweeter functions only for an FM station (and maybe phono - been a long time since I restored an 800-B), which is why the speaker wiring diagram is a bit complicated.
Do carefully read the operating instructions because a control or two must be in specific settings for the radio to switch between FM, AM and short wave. Also, I recall that factory wired only 2 or 3 of the preset buttons for FM at the low end of the dial, but you can rewire any preset button for AM or FM in order to access an FM station anywhere higher on the FM dial.
Some 800-B radios were delivered with a remote control keyboard with a long 20 wire cable to operate the station presets, volume control and switching between radio and phono. Those delivered with the remote control keyboard had an electric motor and reduction gears to turn the volume control. The hard to find remote control key pad with 21 pin Jones plug fits into any 800-B tuner to control the station pre sets, but if the 800-B tuner lacks the volume control motor the keyboard can not control the volume. Also - the pre-war Phantom and Philharmonic keyboards and the post war 800-B keyboards look the same, but are wired differently and, so, pre-war and post-war keyboards are not interchangeable without extensive rewiring.
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Riders volume 15 has extensive documentation and diagrams for the Scott 800-B.
Jensen co-ax photos:
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Wow guys, almost TMI!. It's a future project so I'll have to gather some information before attempting. Is there any information available about wiring to substitute a PM speaker for the EM one?
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Find the male Jones 10 pin plug that fits the speaker socket. Then wire an appropriate speaker with or without a tweeter following the diagram - Riders diagram in Volume 15, pages 41 & 42, assuming your serial number is above number 2000, otherwise pages 39 &40. Diagram shows using the 8 ohm portion of the output transformer for both low frequency and tweeter speakers, and note the crossover made of a cap and resistor network for the tweeter.
The AC- jumper uses pins 9 & 10. The power resistor to sub for the B+ field coil is 675 ohms. Tweeter and main speaker go through the plug separately. The specs show a frequency range on FM audio 35 to 15,000 cycles with the tweeter on FM only. Speaker circuit paragraph explanation on page 45 of vol 15, Riders.
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Where would the serial number be located?
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OK all it took was moving some things around on my project table (it's like those finger puzzles we all had as kids with 25 spaces and 24 numbers except mine has more numbers, but I digress) and I looked at the back of the tuner and found "800 5916", the 5916 I assume to be the SN. Does this indicate a later model? Thanks!
Daniel Kingloff said:Where would the serial number be located?
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Yes, that would be a late model manufactured around January 1947 but likely stored on a shelf until needed to fulfill an order. There will be an inspection date ink stamped on the inside of the station finder commutator shroud (square box on back of tuner chassis) on the left side (looking from the front of the chassis) below the elevation of the top of chassis. One must look down to see it and a flashlight is helpful.
Norman
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Other than the electrolytic caps, are there any other usual suspects that will need replacement?
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